Method of cultivating plants without soil

ABSTRACT

A method of cultivating plants without soil, wherein a nutrient solution is supplied to the root system of the plants. 
     Before the nutrient solution is supplied to the root system it is oxidized with substantially pure oxygen to maintain a content of dissolved oxygen at the root system which is at least 4 ppm.

This application is a continuation of application Ser. No. 302,411,filed Sept. 10, 1981 now abandoned.

The invention relates to a method of cultivating plants without soil,wherein the nutrient solution is supplied to the root system of theplants.

In all cultivation, whether it takes place in earth or soil in theconventional manner or in a nutrient solution which is supplied to theroot system of the plants, the taking up of negative ions (anions) bythe plants takes place primarily by means of the energy which isreleased on respiration by the roots. Since oxygen is used for normalroot respiration, a high oxygen content in the soil water or nutrientsolution is of great importance for supplying the plant with thenecessary nutrients. In this connection reference may be made to anarticle by H. Marschner: Einfluss der Sauerstoff-Versorgung der Wurzelnauf Mineralstoffaufnahme und Pflanzenwachstum, KALI-BRIEFE, Fachgebiet2,2. Folge: 1-17, 1976.

The connection between the root respiration and the absorption ofnegative ions can often be observed in the practical cultivation ofplants. For example, if the roots of young tomato plants suffer from alack of oxygen because of too low an oxygen content in the soil water ornutrient solution, the root respiration becomes more difficult and hencethe ion absorption in general and particularly the absorption of nitrateions. The plants turn yellow and show distinct symptoms of a nitrogenshortage.

In the cultivation of plants without soil, more attention has been paidto the importance of root respiration for the production of the plants.At the beginning of the culture there is sufficient oxygen for the youngroots, and the root respiration then works well but gradually, as theplants become older, a certain amount of root death occurs as the plantsrenew their roots and the old ones die off. These dead roots then use upsome of the oxygen which is available in the nutrient solution, sinceoxygen is used in the decomposition of these dead roots so that theoxygen content in the nutrient solution which is available for theactive roots, is reduced. The reduced supply of oxygen in the nutrientsolution can lead to the fact that the root respiration becomesinadequate and a certain tendency towards root death can then beobserved, with the result that further dead roots now come to use upfurther oxygen in one nutrient solution.

It is also known that the absorption of ammonium ions NH₄ ⁺ by the rootsleads to a lowering of the pH value of the nutrient solution because theroots then give off hydrogen ions H⁺. Bacteria in the nutrient solution(nitrification bacteria) also reduce the ammonium to nitrate using upoxygen in accordance with the following: ##EQU1##

The physiological mechanisms described mean that the cultivation entersa vicious circle in which the oxygen which is supplied to the rootsystem of the plants through the nutrient solution is used otherwisethan to the advantage of absorption of nutrient by the plants.

In the cultivation of plants without soil, in order to introduce oxygeninto the nutrient solution, air was previously blown into this through aperforated pipeline or hose in a supply tank for the nutrient solution,using compressors. This is described, for example, in the Swedishlaid-open specification No. 7510385-3. The oxygen content of thenutrient solution which is brought about in this manner is notsufficient, however, to supply the oxygen which is needed.

With knowledge of the circumstances related above, the object of theinvention is to provide optimum conditions for the plants duringcultivation without soil with the supply of nutrient solution regardingroot respiration and nutrient absorption by the plants and so toincrease the production of the plants.

With this object in view, according to the invention the method definedin claim 1 is proposed and in order to explain the invention anembodiment thereof will be described in more detail below with referenceto the accompanying drawings in which

FIG 1 is a diagrammatic representation of an installation for thecultivation of plants without soil, adapted to use the method accordingto the invention,

FIG. 2 is a graph which shows the content of dissolved oxygen in thenutrient solution, and

FIG. 3 is a graph which shows the solubility of atmospheric oxygen orpure oxygen in water.

FIG 1 shows a cultivation bed 10 which comprises a channel 11 in whichplants are "planted", the plants being held in position by a root lumpof rockwool or a similar inactive substrate while their root system isprotected by a cover, an opaque plastics film or the like, whichexcludes the light from the root system Nutrient solution is supplied tothe channel 11 at 12 and this nutrient solution runs through the channelin which the root system of the plants is spreading, to be lead awayfrom the channel at 13 where the nutrient solution can go to an outletor can again be supplied to the channel at 12 by being caused tocirculate in a closed system.

In the present case, such a closed system is assumed to be provided andthe nutrient solution leaving the channel 11 is supplied to a supplytank 14 for nutrient solution. From this nutrient solution is drawn bymeans of a pump 15 to be supplied, via a sand filter 16, to an oxygensupplier 17 which here comprises a container 18 with an inlet 19 for thenutrient solution at the top. This inlet is connected to an admissionchamber 20 in the container, which is separated from the rest of theinterior of the container by means of a perforated partition 21 having alarge number of holes with a diameter of 4 mm. Disposed below thepartition or grid 21 is an injection opening 22 for the supply of oxygenfrom an oxygen flask 23, and at the very bottom of the container thereis an outlet 24 leading from the bottom. In the oxygen supplier, gaseoussubstantially pure oxygen is introduced into the nutrient solution andis dissolved in this. From the outlet, the nutrient solution enrichedwith oxygen in the oxygen supplier is conveyed to a valve 25 to besupplied in a predetermined quantity to the cultivation bed 10 andotherwise to return to the tank 14.

The closed circulation system for the nutrient solution describednaturally also comprises valves and various by-pass and branch pipes inconventional manner, but these details are not shown here. Thecomposition and acidity of the nutrient solution can be regulated inaccordance with the method of cultivating plants without soil which isdescribed in the Swedish laid-open specification No. 7510385-3.

In FIG. 2 the horizontal axis indicates the temperature of the nutrientsolution in °C. and the vertical axis the amount of dissolved oxygen inthe nutrient solution in ppm. A horizontal line A indicates the criticallimit for the oxygen content in the nutrient solution which, accordingto the invention, has been found to be about 4.5 ppm of dissolvedoxygen. Below this value, the root respiration is inadequate. The curvesB and C indicate the lower and upper limits, respectively, for thevalues of oxygen contents measured in practice during conventionalcultivation of plants without soil, where the only supply of oxygen isobtained by blowing air into the tank in the manner mentioned above Thecurve D indicates the highest possible value of the oxygen content, thesaturation value, with air blown in.

As can be seen from FIG. 2 the capacity of the nutrient solution to holdoxygen dissolved is in inverse proportion to the temperature of thenutrient solution. During warm summer days, the circulating nutrientsolution can rapidly come up to temperatures of 26° to 28° C., and itcan be seen from FIG. 2 that at this temperature level the nutrientsolution can hold at most about 5.5 ppm of dissolved oxygen.

It is therefore an important feature of the method according to theinvention that the amount of dissolved oxygen in the nutrient solutionshould be prevented from dropping below the said critical limit and forthis purpose the installation is provided with means for detecting thecontent of dissolved oxygen in the nutrient solution which is suppliedto the channel 11, and to regulate the oxygen content to a valuesuitable for the plants.

These means comprise a measuring cell 26 of known type in a by-pass fromthe outlet 24 to the tank 14 to measure the oxygen content in thenutrient solution which leaves the oxygen supplier 17 to be supplied tothe cultivation bed 10. The measuring cell 26 is connected to anindicating and control device 27 which indicates the oxygen contentmeasured and is provided with members for adjusting a rated value of theoxygen content. The indicating and control device 27 is coupled to asolenoid valve 28 to regulate the supply of oxygen from the oxygen flask23 to the oxygen supplier 17, this valve being opened pulsewisedepending on the actual value of the oxygen content in the nutrientsolution from the oxygen supplier 17 to keep this value in agreementwith the preset rated value; a deviation between actual value and ratedvalue leads to an increased pulse frequency with regard to the openingof the solenoid valve 28.

Because the supply of oxygen to the nutrient solution takes place usingan oxygen supplier 17, which is a novelty in the cultivation of plantswithout soil and is based on recognition of the fundamental importanceof the supply of oxygen to the root system of the plants for the growthand production of the plants, a large, regulated amount ofoxygen--considerably greater than was possible previously--can beabsorbed by the circulating nutrient solution as dissolved oxygen. Theoxygen content in the nutrient solution naturally drops during thepassage of the nutrient solution through the channel 11, but regardlessof whether a greater or smaller amount of the dissolved oxygen isconsumed during the passage through the channel 11 or whether thenutrient solution has a higher or lower temperature, the describedsystem of supplying oxygen means that the content of dissolved oxygen inthe nutrient solution at the root system is kept at a value which issufficiently far above the critical value of 4.5 ppm to achieve theoxygen-rich milieu for the roots of the plants which is needed in orderthat the ion exchange and root respiration and hence the absorption ofnutrient in the plants may be the most favourable. In order to maintainthis oxygen content it is necessary for the nutrient solution which issupplied to the channel 11 to have a considerably greater content ofdissolved oxygen than can be supplied simply by blowing in air. In thisconnection reference should be made to FIG. 3 which shows the solubilityof oxygen in water, the horizontal axis indicating the temperature ofthe water in °C. and the vertical axis indicating the amount ofdissolved oxygen in ppm. The curve E relates to the solubility ofatmospheric oxygen (20.9% O₂) and the curve F to the solubility of pureoxygen (100% O₂).

How great the oxygen content should be in the outlet 24 from the oxygensupplier 17 depends on the oxygen content which is present at 13 wherethe oxygen content should not be lower than 4.5 ppm. The oxygen contentat 13 depends on many factors, for example the temperature of thenutrient solution, the root activity of the plants, the kind of plant,the length of the cultivation bed 10, the content of organic material inthe nutrient solution and the amount of ammonium nitrate in the nutrientsolution. The nutrient solution which leaves the oxygen supplier 17 can,for example at 20° C., be given a content of dissolved oxygen which isas high as about 44 ppm. Here, too, the oxygen content naturally variesin inverse proportion to the temperature of the nutrient solution. Thevicious circle with progressive decomposition of the plants because ofan increasing lack of oxygen for the root system of the plants can bebroken to the advantage of the production.

The purpose of the sand filter 16 is to separate out particles whichaccompany the nutrient solution leaving the bed 10. Organic particleswhich are caught in the filter are decomposed more quickly with a highoxygen content in the nutrient solution. As a consequence thereofnutrients bound in these organic particles are returned to the system.

The supply of oxygen also compensates, of course, for an initially lowoxygen content in the untreated water which is included in the nutrientsolution.

The oxygen supplier described has proved advantageous for the purposeindicated but it is also possible to use oxygen suppliers of anothertype. What is important is that the oxygen is supplied to the nutrientsolution under pressure and that the oxygen is given time to dissolve inthe nutrient solution under intensive mixing.

I claim:
 1. In a method for the cultivation of plants without soil bysupplying an oxygenated nutrient solution to the root system of theplants, the improvement comprising oxygenating the nutrient solution bypassing the nutrient solution, before it is supplied to the plants, infinely divided form through an atmosphere of substantially pure gaseousoxygen under pressure, maintaining imminent mixing contact between thesolution and the gaseous oxygen sufficient to achieve a dissolved oxygencontent of at least 4.5 ppm in the nutrient solution, continuouslymeasuring the dissolved content of the oxygen in the solution followingmixing and before circulating the oxygen enriched nutrient solution backto the root system of the plants and adjusting the volume of oxygensupplied to the mixing step depending on the quanity of oxygen measured.2. The method of claim 1 in which the nutrient solution is recirculatedin a closed system, the spent nutrient solution leaving the root systembeing pumped through the atmosphere of gaseous oxygen.
 3. The method ofclaim 2 in which the spent nutrient solution is filtered before itpasses through the oxygen.